Any control system comprises one or more controllable members by means of which a process is controlled. A control valve is perhaps the most common example of such a controllable member. Control valves are subject to wear, as is well known to anyone that has been involved in industrial control. After some time in operation, this wear results in friction and backlash that deteriorate the control performance. Therefore, valves have been identified as the major source of problems at the loop level in process control.
Valves with a high level of static friction (stiction) results in stick-slip motion that causes the control loops to oscillate. As the amount of friction increases, so does the backlash in the linkage mechanism in the positioner and actuator of the valve. The backlash adds a time delay to the control loop which deteriorates the control. Since control loops in process control applications often are coupled to surrounding control loops, there is also a risk that the disturbances caused by backlash in one loop will propagate to other loops.
When the stiction or backlash becomes large, the valve should, of course, be repaired or replaced. However, this can normally not be done without interrupting the process. For this reason, and other economical reasons, it is of interest to try to keep the valve running for as long time as possible. Stiction and backlash can be compensated for using available methods, provided that an extent or amount of the stiction or backlash is known to the compensation method used.
Even though the problems caused by stiction and backlash are severe, they are often not discovered by operators in process control plants. The main reason is that the reduction of personnel has resulted in a situation where each operator simply has too many control loops to supervise. Therefore, research on procedures for automatic performance monitoring has been very active in the last decade. The industrial use of such procedures has also increased rapidly in recent years. As regards stiction, a plurality of automatic methods are known for detecting control loops with stiction. When it comes to backlash, on the other hand, no efficient automatic detection or estimation procedure is known in the prior art. Instead, a manual investigation has been required by the process control operator. One example of such a manual investigation according to the prior art will now be briefly described.
FIG. 1 shows a block diagram of a control loop with backlash, the control loop involving a controller C and a process P to be controlled. The controller C receives a setpoint value signal ysp and a process output signal y as inputs, and produces a control signal u as output. The controller output u is not directly input to the process P; rather, it goes through a backlash that gives the true process input ub. FIG. 2 illustrates the function of the backlash, where the dead band caused by the backlash is denoted d. When the control signal u is reversed, the process input ub remains constant until u has passed the dead band d.
The amount of backlash in the control loop of FIG. 1 can easily be determined manually by the procedure shown in FIG. 3. In FIG. 3, the upper diagram shows the process output signal y, and the lower diagram shows the control signal u. The experiment starts with two step changes in the control signal u in the same direction. If the first step is sufficiently large, the effect of the backlash is not present in the second step. The third step is then made in the opposite direction. Now, the control signal u has to pass the whole backlash before the valve moves. If the last two steps are of the same size, the backlash is d=Δy/Kp, where Δy is the difference between the process outputs after the second and the third step (see FIG. 3), and Kp is the static process gain (also easily obtained from FIG. 3). This manual procedure may be used each time a control system operator inspects the control loop.
However, as has already been explained, there is an ongoing trend in various fields of industry to reduce the personnel in process control plants. Consequently, the times between manual inspections of the control loops may often be long. Therefore, it would have been favorable in various fields of industry to be able to detect and estimate the amount of backlash automatically, i.e. without any involvement by a human operator.